1. Field of the Invention
The present invention relates to a piston-operated refrigerant compressor having at least one compressing piston provided with a piston ring fitted therein and to a method of assembling the piston-operated refrigerant compressor and, particularly, to a method of assembling the pistons in the cylinder bores of the compressor.
2. Description of the Related Art
In known refrigerant compressors and, particularly, refrigerant compressors accommodated in a vehicle climate control system, carbon dioxide gas (CO.sub.2 gas) is practically used as a refrigerant gas instead of the conventional fluorinated hydrocarbons gas, to prevent environmental problems. When CO.sub.2 gas is used as a refrigerant gas for the compressor of the vehicle climate control system, the gas must be compressed to a relatively high pressure in order to exhibit a refrigerating performance suitable for cooling air in the vehicle compartment. Therefore, it is required that an annular gap between the cylindrical inner wall of each cylinder bore and the outer circumference of each piston is gas-tightly sealed to achieve an effective compression of the CO.sub.2 gas while preventing an increase in an amount of blow-by gas through the gap. Further, a smooth reciprocation of the piston in the cylinder bore must be maintained. Accordingly, the sealing of the annular small gap between the bore wall of the cylinder bore and the outer circumference of the piston must be achieved by a sealing means more effective than the conventional simple sealing method in which the piston and the bore wall of the cylinder bore is maintained in a snug fitting condition without using any particular sealing elements. Thus, adoption of a piston ring for sealing the gap between each piston and the bore wall of the cylinder bore of the refrigerant compressor, which was not adopted in the field of the conventional fluorinated hydrocarbons gas type refrigerant compressors, has been recently reconsidered.
When a refrigerant compressor uses reciprocating pistons fitted with piston rings in circumferential grooves therein, the outer diameter of each piston ring in a free and non-compressed condition is formed larger than the inner diameter of the bore wall of the cylinder bore to obtain a good sealing performance. Therefore, when the compressor is assembled and when the respective pistons with the piston rings are fitted in the cylinder bores, the piston rings must be compressed to reduce the outer diameter thereof before the pistons and the piston rings are inserted into the respective cylinder bores of a cylinder block. For example, in the case of a single-headed piston type swash-plate-operated refrigerating compressor, when a piston unit including one set of pistons fitted with piston rings and assembled with a swash plate is inserted into the corresponding cylinder bores of the cylinder block, the ends of the respective cylinder bores opening toward a swash plate chamber of the compressor housing are chamfered to have tapered ends through which the pistons with the piston rings are forcedly inserted into the corresponding cylinder bores. In order to smoothly insert the pistons and the piston rings into the cylinder bores, the bore ends must be chamfered to have a large oblique face, respectively. Nevertheless, when the large oblique faces are formed in the respective bore ends, the length of each cylinder bore to smoothly guide the reciprocation of the pistons is reduced to result in a reduction in a reliable reciprocating operation of the respective pistons. Further, if an inclining angle of the oblique face of each bore end is increased to intentionally enlarge the diameter of the entrance of the bore end, the piston ring fitted in the piston cannot be in a smooth sliding contact with the oblique face of the bore end so that the piston and the piston ring are not smoothly inserted into the corresponding cylinder bore. Namely, assembling of the pistons and the piston rings into the cylinder block cannot be effectively achieved. Therefore, it cannot be said that the provision of the conventional chamfered oblique face at the bore end of each cylinder bore of the cylinder block of a refrigerant compressor is effective for improving the assembling operation of the piston unit into the cylinder bores of the cylinder block. Thus, when the piston unit is inserted into the cylinder bores of the cylinder block, the piston rings fitted in the respective pistons must be manually compressed from the outside to elastically reduce the diameter thereof before the pistons together with the piston rings are urged into the corresponding cylinder bores. Accordingly, it usually takes a long time to assemble the piston unit into the cylinder bores of the cylinder block. Particularly, radially inner portions of the respective cylinder bores arranged radially closer to the central bore of the cylinder block in which a drive shaft is mounted, must be arranged closer to one another in a circumferential direction, so that spacing between the neighboring radially inner portions of the two neighboring cylinder bores is considerably small from the viewpoint of the structural requirement of the cylinder block. Thus, the small spacings between the respective two neighboring inner portions of the cylinder bores cause a difficulty in the assembling of the pistons and the piston rings into the cylinder bores while compressing the piston rings either manually or by the use of a specified assembling jig.